Security
The building that Cindy chose to live in had a large door that only residents could open with a special key. It was part of the reason why she wanted to live there because she felt safe and secure with this barrier of entry. But she would always check it twice to make sure it was locked.
It was just a habit that she had developed over the years and she always wanted to know who was coming in and out of the building when she could see. She tried to not get obsessive about this, but her fear of criminals was still very much there. Would she ever get over it?